FRANKFURT. Sulfur hexafluoride (SF₆) is a chemically stable, colorless, odorless, and non-toxic gas. It is used all over the world, primarily as an insulating and protective gas in electrical switchgear within medium- and high-voltage engineering. In the past, it was also used in Germany as a filling gas for sports shoes and as an insulating gas in soundproof windows until this application was banned in 2006.

SF₆ is the most potent greenhouse gas known to date: One kilogram of it contributes as much to global warming as about 24 tons of carbon dioxide (CO₂). Due to its extreme climate impact, the United Nations requires all its member states to regularly report their SF₆ emissions. To date, Germany’s greenhouse gas emissions inventory assumed that the majority of SF₆ emissions today stem from the disposal of old soundproof windows, during which the gas escapes into the atmosphere.

Researchers at Goethe University Frankfurt, in collaboration with European partners, have now demonstrated for the first time that actual emissions from a source in southwestern Germany are likely to be significantly higher than previously estimated. The team led by Professor Andreas Engel from Goethe University’s Institute for Atmosphere and Environment has been operating the measurement station at the Taunus Observatory for several years. Since 2023, the observatory has been part of the international measurement network AGAGE (Advanced Global Atmospheric Gas Experiment). “Of all the European measurement stations in the network, we record the highest SF₆ concentrations at the Taunus Observatory,” Engel reports. “We found this puzzling – especially because the highest values occur during southern airflow.” As part of a study initiated and funded by the German Environment Agency to verify the emissions profile using new data and methods, the phenomenon has now been investigated in greater detail.

Researchers subsequently found even higher concentrations in air samples from the ICOS network (Integrated Carbon Observation System) in Karlsruhe. Using atmospheric transport models and a method known as inverse modeling, they were able to apply this top-down approach to spatially pinpoint the emissions. The results showed that the highest emissions come from the Heilbronn region in southwestern Germany, amounting to approximately 30 tons per year or about one-third of Germany’s total SF₆ emissions. However, the scientists emphasize that these amounts are relatively small compared to global SF₆ emissions of around 8,000 tons, of which China alone contributes some 5,000 tons annually.

“This kind of regional distribution of emissions does not align with previous assumptions that emissions primarily stem from the disposal of old soundproof windows,” explains Katharina Meixner, lead author of the study published in the journal ACS Environmental Science & Technology – Air. “What stands out, however, is that this region is home to the only SF₆ production and recycling facility in Europe that we are aware of.” Meixner emphasizes that “only by understanding where emissions in Germany originate can they be properly accounted for and effectively addressed to reduce greenhouse gas emissions.” Andreas Engel explains: “Previous scientific studies have already shown that emissions during the production, use, and recycling of highly volatile substances are often harder to avoid and therefore higher than previously assumed.”

Through its research, the Frankfurt-based group contributes to validating and complementing bottom-up assessments of emissions, which have so far been primarily based on theoretical assumptions, with top-down emission estimates derived from atmospheric measurements. In addition to SF₆, the research group also measures many other halogenated greenhouse gases and ozone-depleting substances.